Many retinal neurons are coupled via electrical synapses or gap junctions composed of specific neuronal connexins from a gene family of about 20. Only recently, Cx36 gap junctions were shown to be essential for the transfer of ON signals in the dark-adapted retina (Deans et al., 2002). Now, we propose to investigate two novel neuronal connexins and describe the role of gap junctions in several additional retinal circuits. Specific Aim 1 is to identify horizontal cell connexins. In the rabbit retina, there are two kinds of horizontal cell, which have different dye coupling properties. We have already shown that A-type horizontal cells use Cx50. The somatic dendrites and axon terminals of B-type horizontal cells form two additional networks. Preliminary data indicate that Cx57 is found in the OPL and we will test the hypothesis that B-type horizontal cells express Cx57. These methods will be extended to the primate retina to test the hypothesis that H1 or H2 horizontal cells use Cx57. In specific aim 2, we will test the hypothesis that S1 amacrine cells use Cx45. In addition, we will test the hypothesis that Cx36/Cx45 pairs form heterotypic gap junctions between All amacrine cells and ON bipolar cells. These gap junctions have been shown to be essential in the function of rod pathways. We have identified the population of melanopsin containing ganglion cells in the rabbit and developed an antibody to target them in live retina. In aim 3, we will test the hypothesis that the melanopsin ganglion cells are connected via gap junctions which are used to synchronize firing. This proposal also has clinical significance. For example, All amacrine cells and the expression of Cx36 are reduced in Parkinson's disease. Gap junctions have also been implicated in photoreceptor degeneration via a mechanism known as the bystander effect. Recent results suggest that hemichannels (or half gap junctions) may open to function as a death channel, mediating ischemic neuronal death, after damage caused by stroke. Finally, many of the fluorescent probes developed in this work will be useful as neuronal markers to follow changes due to retinal degeneration or implants or to identify the source of intraretinal tumors.